SAW components use acoustic waves which travel at the speed of sound. The SAW components are preferred over widely used transmission line components because acoustic waves have a substantially shorter wave length at operating frequency than electromagnetic waves which travel at the speed of light. Therefore, for a given operating frequency, SAW devices provide a smaller structure than a transmission line structure, making them suitable for miniaturized radio frequency applications. Furthermore, SAW devices are integratable with other active circuits, such as amplifiers and mixers, which are produced using conventional integrated circuit technologies. For the above reasons, the popularity of SAW structures in radio frequency applications, especially in resonator filter applications, has been steadily increasing.
FIG. I depicts a conventional SAW resonator structure 100 which includes a SAW transducer 110 and a pair of reflectors 120 disposed on a piezoelectric substrate 105. As is well known, the reflectors increase the quality factor of the SAW resonator by preventing dissipation of surface acoustic waves emanating from the SAW transducer 110 near the resonant frequency. The SAW transducer 110 comprises a first electrode 112 having a first set of open-ended fingers 114 and a second electrode 116 having a second set of open-ended fingers 118. The first electrode 112 and the second electrode 116 comprise conductive layers patterned on the piezoelectric substrate such that a first set of fingers 114 and a second set of fingers 118 are interdigitated in relation to each other. Conventionally the substrate 105 is made of a material with low temperature coefficient, such as quartz. As such, the SAW resonator is used in applications where stable, high frequency (within 100 MHz-1000 MHz range) source is desired.
Electric oscillators are extensively used in many applications where there is a need to generate one or more predetermined frequency output. For example, in frequency modulation (FM) radio frequency (RF) communication, RF receivers and RF transmitters use oscillators for generating carrier frequencies. Generally, oscillators operate at a particular resonant frequency. An oscillator's resonant frequency is determined by reactive elements of a tank circuit which comprises capacitive and inductive components. In conventional voltage controlled oscillators (VCOs), a variable reactive element in the tank circuit controls the resonant frequency of the oscillator. Often, the variable element is a two-terminal, nonlinear capacitor, such as a semiconductor varactor, which is responsive to a control signal for controlling its capacitance. Such tank circuit resonators provide a very wide tuning range (i.e. in the range of 3 MHz to 30 MHz) which makes them particularly suitable in frequency synthesized land-mobile communication applications.
Conventionally, transmission lines and coaxial distributed structures have been used as a substitute for the inductive component of the tank circuit. However, the fundamental companion element to the variable capacitor has relied upon magnetic field storage to provide the inductive reactance necessary to resonate the oscillator with the variable capacitor. As the inductor's size is reduced, the resonator's quality factor, Q, decreases. Decreased quality factor, has a number of undesired consequences, namely, degradation of VCO's sideband noise performance and desensitization of the receiver in the presence of a strong adjacent channel carrier signal. Additionally, because an inductor's magnetic field is very difficult to shield, VCO generated signals, spurious or otherwise, are undesirably coupled to the surrounding circuit.
Another problem frequently encountered in conventional VCOs, is a phenomenon known as "microphonics" which adversely affects an FM receiver's performance. Microphonics is a phenomenon whereby mechanical vibrations around the VCO structure are picked up by the electromagnetic inductor, thereby changing its effective inductance. As such, the resonant frequency of the VCO is changed. Since, in FM systems, the frequency changes are demodulated, the resonant frequency changes are manifested as undesired hum and noise which adversely affect the receiver's audio output.
Historically, SAW resonators have not been used as oscillators in land-mobile communication because it is extremely difficult to get the resonant frequency to change, over a wide tuning range, with a variable capacitor. Typically, the tuning range of for a 900 MHz SAW resonator has been in the range of only a few kilo-hertz, whereas, in land-mobile applications, a typical VCO in that frequency range must tune in megahertz range. In a related co-pending U.S. patent application entitled "A Tunable Oscillator Having A Surface Acoustic Wave Transducer", filed concurrently herewith and assigned to assignee of the present application, which is hereby incorporated by reference, the applicant of the present invention has disclosed a two- terminal SAW device structure which uses inductive properties of a SAW resonator to provide a substantially wide tuning range in an oscillator circuit. This two-terminal SAW device is coupled to external components which receive oscillation feed back from an active device. In some conventional oscillator designs, the oscillation feed back from the active device is fed into an electromagnetic three terminal device, such as a tapped inductor. Having described the problems associated with electromagnetic device, there exist a need for a three-terminal SAW device which behaves as a tapped inductor.